A tool clamping device having these features is described in DE 199 40 330 C2. In this tool clamping device the tool holder has at least two adjustment elements, which are arranged at a distance from one another and protrude axially beyond the support face of the tool holder, whereas at least two bearing points associated with the adjustment elements are formed on the tool carrier and are arranged on the wall of at least one receiving groove formed on the tool carrier, the adjustment elements bearing at said bearing points when the tool holder is inserted into the socket. The adjustment elements in one embodiment are T-nuts, which are guided without clearance on the tool holder and are under the influence of spring means, which, when the tool holder is inserted into the socket, press with a predetermined bias against the wall of the respective receiving groove. The adjustment elements can be adjusted exactly in the sense of the accurately dimensioned adjustment of the tool holder with respect to the axis of the socket of the tool carrier, such that the tool holder can be positioned with high accuracy with respect to the tool carrier when said tool holder is inserted into the socket of the tool carrier. The adjustment elements of the tool carrier are generally pre-set by means of a template corresponding exactly to the support face and the receiving groove of the tool carrier. Following this pre-setting, they can be inserted into a socket of the tool carrier with high repeat accuracy, for example when a tool is changed.
The tool holder, which for example is pre-set in this way, is fastened to the tool carrier by means of four threaded bolts, which are arranged around the axis of the tool holder, distributed in corresponding through-bores in a flange-like part, and are screwed to the tool carrier with the tool holder fitted with its support face against the support face of the tool carrier. The threaded bolts are received in the bores of the flange-like part of the tool holder with radial clearance, such that, when tightening the threaded bolts, no transverse forces are exerted onto the tool holder, which forces could impair the pre-setting of the adjustment elements.
Tool clamping devices of this type have proven to be excellent in practice and have become widespread, in particular in CNC turning centres with movable tool carriers, for example in the form of turret plates. The tool clamping device makes it possible to achieve a very high level of positional accuracy of the tool holder on the tool carrier, more specifically both in terms of the angular accuracy, i.e. the rotary position with respect to the axis, and in terms of the positional accuracy, i.e. of the axial offset between the socket of the tool carrier and the inserted tool holder.
In some applications the user increasingly desires a reduction in the tooling time necessary to change a tool. The need for a quicker tool change is given here from the increasingly smaller batch sizes, which force the user to make modifications more frequently in order to thus ensure that the machines are working to a profitable capacity. What are known as quick-change systems exist generally in the field of workpiece clamping and make it possible for workpieces, which for example are fixed on pallets, to be fitted in a defined position on a carrier body arranged on a machine bench and securely clamped to said carrier body by means of a clamping apparatus that can be actuated easily and quickly, generally from an actuation point. A unit for detachably connecting parts of a palletising system is thus described in EP 0 922 529 A1, which unit operates with a pressure-medium-actuated, centrally arranged bolt, which engages with an associated central bore in the part to be clamped and exerts an axial clamping force onto said part during the clamping operation, under the influence of which force this part is engaged with conical centring faces on another part. Such clamping devices, which operate with a central clamping bolt, are therefore not suitable for fastening tool holders for example on a turret plate, because the tool holders often have a standardised cylindrical shaft, with which they are inserted into a receiving bore arranged centrally on the turret plate relative to the support face. The same is also true fundamentally for clamping devices that are similar in principle, which are provided with a centrally arranged draw-in nipple, which transmits the axial clamping force, for a pallet or clamping plate for mounting workpieces, as is described for example in DE 197 05 684 A1 and EP 0 858 861 and DE 199 17 9005 C1.
In a quick-change apparatus known from U.S. Pat. No. 5,167,405 A for workpieces clamped on a workpiece carrier, the workpiece to be machined is clamped on a workpiece carrier upper part by means of clamping jaws. The workpiece carrier upper part formed as a rectangular plate is fixedly screwed in the vicinity of its corners to four cylindrical tension bolts, which protrude through comparatively large openings into a box-shaped workpiece carrier lower part and are supported in the clamping state on the bottom of said lower part. A clamping apparatus accommodated in the workpiece carrier lower part has a rotatably mounted shaft, which is provided with an actuation handle and which carries an eccentric, via which, when the shaft rotates, four clamping bolts are simultaneously brought into engagement with the four tension bolts of the workpiece carrier upper part. In order to align the workpiece carrier upper part with the workpiece carrier lower part, two guide pins arranged spaced apart from one another on the workpiece carrier upper part are provided and are received in corresponding bores in a cover plate of the workpiece carrier lower part. The positioning accuracy of the workpiece carrier upper part attainable with such a quick-change apparatus is limited to the workpiece carrier lower part. There is no precise adjustment of the mutual position of the two parts, and this also is not possible. Much higher demands are placed on the positioning accuracy by a clamping device having repeat accuracy, in particular for clamping workpieces for machining, as is disclosed in DE 101 55 077 A1. This clamping device operates with a reference plane plate, which has at least two clamping openings having walls, on each of which at least one reference face is formed. The reference plane plate is associated with a clamping plate, which has at least two clamping and positioning bolts, which are associated with the clamping openings. A tightening device is provided in order to act on the clamping and positioning bolts with an axial force and a superimposed radial force in order to press a base area of the clamping plate against a clamping face of the reference plane plate by means of the axial force and in order to push the clamping and positioning bolts with positioning faces against the reference faces by means of the radial force. This clamping device does not have any means for adjusting the position of the clamping plate relative to the reference plane plate. The attainable accuracy is dependent on the positional accuracy of the clamping openings.